1. Field of the Invention
The present invention relates to a pneumatic tire with good fuel efficiency, drainage performance and uneven wear resistance, especially to a tire for a heavy load vehicle such as a truck and a bus.
2. Description of Related Art
A pneumatic tire, especially a heavy load pneumatic tire for a heavy load vehicle such as a truck and a bus may have a so-called rib-like pattern where, as shown in FIG. 6, a tread surface is provided with a plurality of circumferential main grooves extending along the circumferential direction of the tire to define a plurality of rib-like land portions on the tread surface, or a so-called block-like pattern where a tread surface is provided with a plurality of circumferential main grooves extending along the circumferential direction of the tire and widthwise main grooves extending along the widthwise direction of the tire to define a plurality of block-like land portions on the tread surface.
In the tire having a rib-like pattern, the total opening length of provided grooves and sipes is small, and thus edge component resided in the ground contact area is small, whereby an improvement in wet performance by virtue of an edge effect cannot be expected. Nevertheless, it is possible to increase the widths of circumferential main grooves to secure drainage performance. With respect to a reduction of rolling resistance contributing to fuel efficiency, since the rib-like land portions integrally extend along the circumferential direction of the tire, stiffness of the land portions against input forces in the circumferential direction of the tire that act during driving and/or braking of the vehicle increases, and thus the deformation of the land portions can be suppressed; however, since the rib-like land portions are divided from each other by the circumferential main grooves with relatively large widths, the input forces in the widthwise direction of the tire that act during cornering or the like of the vehicle tends to increase the deformation of the land portions in some extent. Therefore, the overall rolling resistance cannot be sufficiently suppressed. In addition, regions of rib-like land portions located adjacent to the circumferential main grooves with relatively large widths have larger local shear deformation in the ground contact area and suffer from enhanced wear, which arises a problem that the regions become cores of uneven wear.
As a tire having a rib-like pattern intended to improve drainage performance as well as reduce rolling resistance, for example, JP 2000177333 A describes a pneumatic tire having a tread pattern in which rib-like land portions are provided with a plurality of the widthwise narrow grooves having a width of 0.5-1.5 mm and extending along the widthwise direction of the tire to dividingly form block-like land portions; and the block-like land portions are provided with closed sipes inclining in the direction opposite to the widthwise narrow grooves with respect to the axial direction of the tire.